The present invention relates to functionally graded materials (FGM) and methods of making same, and more particularly to making functionally graded materials via slurry processing methods.
The goal of most ceramic forming processes is to create a green body that is as homogeneous as possible, so that the structure will be very uniform after sintering. It is well known that even minor inhomogeneities in the green body can lead to stresses, distortions, and poor mechanical properties (even fracture) of the object after sintering. For example, during slip casting the movement of water from the liquid slurry into the porous mold creates small nonuniformities in the green density of the component and can lead to residual stresses after firing. One of the major benefits of the ceramic gelcasting process is that a uniform slurry is created and poured into a mold, where gelation of the liquid vehicle locks the constituent powder particles into place before settling, floatation, flocculation, or other source of inhomogeneities can operate.
On the other hand, there are many applications in which it is desirable to have a dense ceramic and/or metal component in which the properties vary in some way throughout the article. It may be desirable to produce an article characterized by high hardness on one surface and high toughness in the interior, for example. Particularly for uses involving microwave energy, it is often desirable to have the loss tangent and dielectric constant of an article vary so that microwave energy can be effectively absorbed by the article while minimizing reflections.
Accordingly, objects of the present invention include provision of methods of making metallic, ceramic, and/or composite articles having compositions that are graded spatially within the articles so that one or more physical properties of the finished articles vary in some desired way therethroughout. Such methods include provisions for minimizing the effects of variable properties of the constituents, such as sintering characteristics. Further and other objects of the present invention will become apparent from the description contained herein.
In accordance with one aspect of the present invention, the foregoing and other objects are achieved by a gelcasting method of making an internally graded article which includes the steps of:
a. preparing a slurry including a least two different phases suspended in a gelcasting solution, the phases characterized by having different settling characteristics;
b. casting the slurry into a mold having a selected shape;
c. allowing the slurry to stand for a sufficient period of time to permit desired differential settling of said different phases in order to achieve a vertical compositional gradient in the molded slurry;
d. gelling the slurry to form a solid gel while preserving the vertical compositional gradient in the molded slurry;
d. drying the gel to form a dried green body; and
e. sintering the dry green body to form a solid object, at least one property thereof varying along one direction because of the vertical compositional gradient in the molded slurry.